Advice? Tacho adapter - 12v PWM signal -> coil signal tacho

[j.k.QV]

Short intro: I've managed to put an Alfa 155 8v TS engine together with ECU/relays/loom into an Alfa 75TS. The engine runs (although not developing full power yet - still working on it).

The problem I have now is that 155 ECU (bosch m1.7) gives a 12v square wave signal (30Hz/12V @ 900rpm, 100Hz/2V @ 3000rpm) as tach output, but the 75TS tacho expects induction coil tach signal. As far as I understand I could make a tach adapter using transistor, coil and some resistors?
But how do I calculate what kind of parts would I need? Any suggestions?

EDITED: the signal is 12V, not 2V as I thought before

[festy]

Use a generic Hella style relay for the coil, but open it up and remove the spring and contact arm to stop it buzzing.
Diode is any typical 1N4xxx, and whatever NPN transistor you have on hand.
The resistor value also isn't critical, something around 2k should be close enough.

This was the first schematic I found when searching for "tacho booster relay":

coilnegemulator4ly.gif (18.61 KiB) Viewed 7402 times

[j.k.QV]

Built the tacho booster yesterday, but it doesn't seem to work. Don't have a scope yet to check what's going on exactly. I used a 1N4007 diode, 2N4401 NPN transistor,a 2K2 resistor and a generic automotive relay as a coil. could it be that the 2V signal is too low to switch the transistor through the diode?

[festy]

Are you sure it's 2v? I would have expected a 5v pulse

[j.k.QV]

Are you sure it's 2v? I would have expected a 5v pulse

Ah - crap - you're right. The 2V/20ms is oscilloscope recommended settings, not actual voltage of signal...
For the signal itself it says just 30Hz @ idle (900rpm). And this is the signal that should be there (if the scale is right... it should even be a 12v DC pulse ?):

tach_signal.png (182.27 KiB) Viewed 7351 times

P.S. I bought a cheapie oscilloscope that should arrive next week, so I might as well tackle this when I can see the signal.

[Circlotron]

I'd leave that diode out altogether. It doesn't serve any purpose seeing the 5V drive signal doesn't swing below ground. In fact, leaving it out should make the transistor turn off quicker, giving a cleaner, more sharply defined output pulse. A BC639 transistor from Jaycar would be a good fit, rated at 80V 1A - just watch the pin connections, they are different from most.

Edit -> maybe the diode is there just in case the drive signal doesn't go all the way to round. Gives an extra 0.6V drop or so. maybe leave it in after all. If you can't get any joy, when your scope turns up check the dive signal is actually going down close to ground.

[j.k.QV]

I connected a multimeter (finally ) and it gives ~12v. Will wait for the scope and check the signal to be sure what I'm working with.

[j.k.QV]

Finally got it working - the main problem was - I was using the wrong source wire all along. The actual wire color used was different to the one stated in manual.
Anyways the cheapo oscilloscope (~20Eur) came in handy. Found the signal in the same connector, but different wire.


The signal converter is properly ghetto


Here is a working product demo
https://www.youtube.com/watch?v=fU9JmBr ... e=youtu.be

[OZ38]

Well done Sir. Wish I could work mine out now on my GMH V6 engine Hilux. Seems Marks 4WD Adapters use the crank angle sensor wire (Blue/yellow) from the DIS to their box of tricks instead of the Tacho signal wire (Brown).
Its all wired up correctly it seems but I have no idea of how to test any of it to fault find the issues with why the Tacho doesn't operate.

Cheers

[Jayme]

the last one I made, I tore off the cover for the relay, and put the transistor and resistor inside the casing, then put the lid back on and glued. makes for a nice neat enclosed unit. as long as you remember its not really a relay anymore